Hemispheric cooperation is traditionally considered to be the ability to share information between the hemispheres. We propose that in addition to "hemispheric sharing" there is a second kind of hemispheric cooperation "hemispheric shielding". It involves restriction rather than sharing of information and permits the two hemispheres to work on tasks in parallel, with minimal interference. Preliminary data indicate that hemispheric shielding undergoes a maturational spurt around age 10/11. A longitudinal study of children who are initially ages 9, 10, 11 will be conducted. We will test the hypothesis that the development of hemispheric shielding represents a distinct neuropsychologicla milestone dissociable from the age of greatest improvement in: hemispheric sharing, general inhibitory ability, and speed of responding. Adults will be tested to provide a standard. We also postulate that the degree of hemispheric shielding and/or sharing is related to general scholastic aptitude, vocabulary, reading fluency, reading comprehension, and/or arithmetic ability. We also expect superior readers to show precocious hemispheric shielding and/or sharing. Testing will include visual and motor measures of hemispheric shielding and sharing, two tests of general inhibitory ability, and estimates of arithmetic ability and fluency of reading. Scholastic aptitude and reading scores will be available from the school records. These findings will increase our understanding of reading and its dysfunction; provide a new conceptual framework for study of hemispheric cooperation; and identify a neurological correlate of the cognitive shift which occurs between the ages of 10 and 12.